(1) Field of the Invention
This invention relates generally to high-speed amplifiers and relates more particularly to buffer amplifiers used in communication devices to amplify the signals of a voltage-controlled oscillator (VCO) and to isolate said VCO from a load connected to said amplifier in a frequency range of GHz as for e.g. the Bluetooth technology.
(2) Description of the Prior Art
Modern wireless communication devices use high-frequency signals: 900 MHz to 1900 MHz for cellular phones and higher, up to 6 GHz, for other systems, such as wireless LANs. For example, radios for the “Bluetooth” standard” operate in the unlicensed Industrial, Scientific and Medical (ISM) band at 2.4 GHz. Signals at such frequencies are difficult to generate and to control. They have also a tendency to interfere with each other, as they are easily coupled by parasitic properties present in all electronic components, including integrated circuits
A buffer amplifier is designed to follow low-level stages. This will then present a sufficiently high enough input impedance so it is not considered a significant load to that stage. The intermediate or buffer amplifier stage, while not representing a load must then have a sufficiently low output impedance to drive successive stages. One example for such a low level stage is a voltage-controlled oscillator. An oscillator for optimum performance cannot be loaded down, it needs an intermediate stage following. This will then present a sufficiently high enough input impedance so it is not considered a significant load to the oscillator. The intermediate or buffer stage, while not representing a load must then have a sufficiently low output impedance to drive successive stages
A buffer amplifier is often used to amplify the signals of a VCO and to isolate the VCO from a load connected to the buffer amplifier. For a usage in the GHz frequency range the reverse biased (output to input) isolation should be in the range of 20–30 dB. The quality of said isolation has direct influence on the quality of the VCO signals. The power consumption of said buffer amplifier is another important attribute. Designing such a system is still a challenge for the designer.
U.S. Pat. No. 6,163,222 (to Kobayashi) teaches a buffer amplifier to be connected to an oscillator for outputting an oscillation signal, comprising an amplifier circuit for amplifying the level of the oscillation signal and frequency selective means connected to the output terminal of the amplifier circuit, wherein the fundamental harmonic or the secondary higher harmonic of the oscillation signal is outputted by switching the frequency selective means.
U.S. Pat. No. (5,714,914 to Zhou) discloses a high power, low noise voltage-controlled oscillator (VCO) in a transmitter such as a cellular radiotelephone. The VCO includes a resonant circuit, an active part connected to the resonant circuit, and a buffer amplifier, connected to the active part, for isolating the VCO from a load connected to the buffer amplifier. The buffer amplifier is a linear amplifier including a transistor, and the active part and the buffer amplifier may be connected by a capacitance for reducing coupling between the active part and the buffer amplifier.
U.S. Pat. No. (5,960,334 to Nakano) shows an amplifier for amplifying a local oscillation signal of a receiving section or a carrier signal of a transmitting section and a portable telephone apparatus using this amplifier. Said amplifier comprises an amplifying transistor, a parallel resonance circuit, a bypass capacitor and a switching means for switching between a connected state and a disconnected state of a connection point.